Internal combustion engines, particularly those of the reciprocating piston type, currently propel most vehicles. Such engines are relatively efficient, compact, lightweight, and inexpensive mechanisms by which to convert highly concentrated energy in the form of fuel into useful mechanical power. Such an engine is typically operated through a wide range of speeds and a wide range of loads and typically at an average of approximately a fifth of its maximum power output. A power-split hybrid transmission may be employed to make more efficient use of an internal combustion engine in a vehicle by allowing the engine to operate closer to its peak in efficiency or its peak in power, by virtue of continuously variable gear ratio and the assistance of an energy storage device such as a battery.
A power-split hybrid transmission can use what is commonly understood to be a “differential gear set” to achieve a continuously variable torque and speed ratio between input and output. An electrically variable transmission can use a differential gear set to send a fraction of its transmitted power through a pair of electric motor/generators. The remainder of its power flows through another, parallel path that is all mechanical and direct, of fixed ratio, or alternatively selectable.
One form of a differential gear set, as is well known to those skilled in this art, may constitute a planetary gear set. Planetary gearing is usually the preferred embodiment employed in differentially geared inventions, with the advantages of compactness and different torque and speed ratios among all members of the planetary gear set. However, other types of differential gear sets may be used, such as bevel gears other gears in an arrangement where the rotational speed of at least one element of a gear set is always a weighted average of speeds of two other elements.
A two-range, compound-split, hybrid electro-mechanical transmission having four fixed ratios is disclosed in U.S. Pat. No. 6,953,409, commonly assigned with the present application, and hereby incorporated by reference in its entirety. Such a transmission utilizes an input means to receive power from the vehicle engine and a power output means to deliver power to drive the vehicle. First and second motor/generators are connected to an energy storage device, such as a battery, so that the energy storage device can accept power from, and supply power to, the first and second motor/generators. A control unit regulates power flow among the energy storage device and the motor/generators as well as between the first and second motor/generators. The transmission may be operated both in an input-split mode and a compound-split mode. Several clutches of substantial capacity are required, including multiple clutches effecting alternative power flows from a differential gear set to the output of the transmission.
“Input-split” means that one member of a differential gear set is connected to the input member and receives all of the power flowing into the transmission, so that differential gear set can split that power into an electro-mechanical power path including a pair of motor-generators, and a mechanical power path through the transmission. “Output-split” means that one member of a differential gear set is connected to the output member and receives all of the power flowing out of the transmission to the output member, so that differential gear set can combine power from an electro-mechanical power path and a mechanical power path through the transmission. “Compound-split” means that these characteristics of input-split and output-split are both present in the transmission at the same time.
Operation in first or second variable-speed-ratio modes of operation may be selectively achieved by using clutches in the nature of first and second torque transfer devices. In the first mode, an input-split speed ratio range is formed by the application of the first clutch, and the output speed of the transmission is proportional to the speed of one motor/generator. In the second mode, a compound-split speed ratio range is formed by the application of the second clutch, and the output speed of the transmission is not proportional to the speeds of either of the motor/generators, but is an algebraic linear combination of the speeds of the two motor/generators. Operation at a fixed transmission speed ratio may be selectively achieved by the application of both of the clutches. Operation at fixed transmission speed ratio may be selectively achieved by the application of one of the first and second clutches, along with application of either of a third or fourth torque transfer device. The transmission incorporates one mechanical point in its first mode of operation and two mechanical points in its second mode of operation. As used herein, a “mechanical point” is a finite, non-zero speed ratio between the transmission input member and the transmission output member, where the speed of one of the motor/generators is zero. Thus, a mechanical point occurs when either motor/generator is stationary at any time during operation of the transmission.
U.S. Pat. No. 6,527,658, issued Mar. 4, 2003 to Holmes et al, commonly assigned with the present application, and hereby incorporated by reference in its entirety, discloses an electrically variable transmission utilizing two planetary gear sets, two motor/generators and two clutches to provide input-split, compound-split, neutral and reverse modes of operation. Both planetary gear sets may be simple, or one may be individually compounded. A control unit regulates power flow among an energy storage device and the two motor/generators. This transmission provides two ranges or modes of electrically variable transmission (EVT) operation, selectively providing an input-split speed ratio range and a compound-split speed ratio range. One fixed speed ratio can also be selectively achieved, but only by the simultaneous use of multiple clutches.
A single range, input-split system requires a high portion of the engine power to flow through the electro-mechanical path during acceleration, thus increasing motor power requirements and potentially requiring a DC-DC converter between a storage battery and the motor/generators in order to provide enough voltage. Power flow through the transmission would be entirely through the electro-mechanical path with the output member at rest, but the power transmitted with the output member at rest is actually zero. As the vehicle accelerates from rest, the amount of power transmitted through the electro-mechanical path builds up to a relatively high value, then drops to zero as the transmission ratio reaches the mechanical point of the input-split mode of operation. If the vehicle operates at a ratio beyond the mechanical point, such as a low engine speed and high vehicle speed for cruising, the power transmitted through the electro-mechanical path is negative, from the output member toward the input member, that is, counter to the overall flow of power through the transmission, and rises sharply as the ratio changes further from the mechanical point. Additionally, the output motor in an input-split design must also have a high torque rating; typically, one to two times the engine torque, depending on the gear ratio available. The situation with a single range, compound-split transmission is similar, except that the electro-mechanical power flow at low output speeds, below the lower of the two mechanical points, is counter to the overall power flow through the transmission and rises rapidly with falling output speed.
A two range, input-split and compound-split system generally achieves a wide ratio of coverage, but requires clutches for transitions from range to range, with associated spin losses and accessory power requirements to operate those clutches.
Most EVTs constructed to date have a single operating range: that is, a single arrangement of gearing wherein all of the mechanical power must be transmitted through the mechanical path of the transmission at only one ratio (in the cases of input-split or output-split) or two ratios (in the case of compound-split) of input speed to output speed. Moreover, EVT operation may result in a flow of power through the electro-mechanical path that opposed the flow of mechanical power through the mechanical power path at all speed ratios beyond the pure mechanical ratio, that is, the mechanical point. Thus, to cover a wide range of speed ratios from input to output, the single-range EVT must have a mechanical point near the extreme ratio that will be used continuously, such as vehicle cruising, and thus must be able to transmit a large fraction of the total power through the transmission by means of the electric motor/generators during acceleration, necessitating motor/generators with high torque ratings and thus relatively high cost.